The most common, and perhaps the only commercial process employed today to obtain edible corn oil from corn germ involves expression of subtantially all of the oil from the germ by means of a screwpress, optionally followed by extraction of the remaining oil from the press cake using an organic solvent. Similar processes are generally employed to recover the oil from other oil-bearing vegetable materials such as cottonseeds, soybeans, and coconuts.
Oils obtained by means of expression, with or without subsequent solvent extraction, are characterized by a rather dark brown color, a strong flavor, and undesirably high amounts of free-fatty acids, phospholipids, etc. These oils must be subjected to extensive and costly refining processes to remove the impurities and render them suitable for food use.
It has long been assumed that many of the impurities in crude (i.e., unrefined) vegetable oils result from the high temperatures (up to about 150.degree. C.) to which they are subjected during the conventional process, and this, plus the detrimental effect of the conventional process upon the quality of the protein contained in the vegetable materials and the hazards and costs involved in solvent extraction, has for many years led workers to search for practical methods to obtain vegetable oils employing relatively low temperatures and using water as the extraction medium.
As early as 1943, F. B. Lachle, in U.S. Pat. Nos. 2,325,327 and 2,325,328, disclosed and claimed a process for extracting oil from vegetable and animal materials comprising milling the oil-bearing material, in the presence of water, in a ball mill or similar device to "substantially cellular form" in order to liberate the oil from the oil cells.
Lachle exemplifies several oil-bearing starting materials including corn germ. It is clear, although not expressly stated, that the corn germ used by Lachle was dry germ, probably obtained via the dry-milling process.
Apparently, the Lachle process has never been used commercially for the recovery of corn oil or other oils. This may be because Lachle clearly teaches the necessity of milling to an exceedingly fine degree, i.e., to "substantially cellular form", which is a time- and energy-consuming operation.
The first commercial aqueous low temperature process for recovering lipid material is the process developed by Israel Harris Chayen, which has been widely reported in patents and other publications, e.g., U.S. Pat. No. 2,828,018. This process, which was first developed for recovering fat from bones or other animal waste products, basically involves subjecting the material, in the presence of water, to intense impacts, as by means of a hammer mill, removing the solids, and finally separating the fat and water.
When the process is applied to animal products, fat and water separation is a relatively easy matter, because most of the fat will rise to the surface during a settling operation. However, attempts to apply it to vegetable materials have invariably presented problems in the formation of complexes of the oil with the protein present and/or the formation of oil-in-water emulsions which are extremely difficult to break.
Other reported aqueous extraction processes and modifications have certain features in common. They generally involve milling the raw material without any water being added. Milling in the presence of water is said to result in undesirable emulsion formation. After milling, water (usually alkaline water, at a pH of about 10) is added to extract the oil and the solubilized protein. The solid and liquid phases are then separated, as by centrifugation or filtration, and the pH of the liquid phase is lowered to precipitate out and recover the protein. The remaining liquid phase, consisting of an oil-in-water emulsion, is then treated to break the emulsion, and the oil is finally recovered by centrifugation.
These processes are generally characterized, partially because of the problem of emulsion formation, by (1) numerous processing steps, (2) the use of expensive and energy-consuming equipment, and/or (3) one or more chemical additions, such as the adjustment of pH during the process. We have developed a process for recovering an exceptionally high quality crude corn oil involving a minimal number of processing steps, using equipment having relatively low energy requirements, and requiring no chemical additives.